1. Field of the Invention
This invention relates to square wave inverters which receive a DC voltage and generate therefrom a square wave AC output voltage.
2. Description of the Prior Art
Inverter circuits, which convert DC input voltages to AC output voltages, are well known in the art. An example of such an inverter circuit is disclosed in U.S. Pat. No. 4,051,445 entitled "Inverter Converter Circuit for Maintaining Oscillations Throughout Extreme Load Variations", issued to Robert J. Boschert. This patent is hereby incorporated by reference. FIG. 1 illustrates a power supply in accordance with the prior art Boschert patent. Referring to FIG. 1, an inverter 10 includes an output transformer T1 and a saturable core transformer T2. A DC power source 11 is coupled to the center tap 12 of output transformer T1 which in turn is coupled to a winding L1 and a winding L2 of transformer T1. Winding L1 is connected to the collector of a transistor Q1, the emitter of which is coupled to ground through a winding L7 of saturable core transformer T2. When transistor Q1 is on, current flows out of power source 11, through winding L1, through transistor Q1, through winding L7, and to ground. Similarly, winding L2 of transformer T1 is coupled to the collector of a transistor Q2, the emitter of which is coupled to ground through a winding L8 of saturable core transformer T2. When transistor Q2 is on, current flows out of power source 22, through winding L2, through transistor Q2, through winding L8, and to ground.
During operation, transistor Q1 turns on while transistor Q2 is off, then transistor Q1 turns off and transistor Q2 turns on. In this way, a square wave is provided across the primary winding of transformer T1 and hence across the secondary winding L3 of transformer T1. Base drive current for transistors Q1 and Q2 are provided by windings L5 and L6, respectively, of saturable core transformer T2. Specifically, when transistor Q1 turns on, current through winding L7 causes current to flow through winding L5. The current through winding L5 provides base drive current for transistor Q1. When base drive current is flowing through winding L5, there is a voltage across winding L5 equal to the voltage across a resistor RB1, the voltage across the base emitter junction of transistor Q1, the voltage across winding L7, and the voltage across diode D1. As is well known in the art, when a voltage is placed across the winding of a saturable core transformer, the transformer eventually saturates, and the voltage across the winding drops to zero. When the voltage across winding L5 drops to zero, transistor Q1 turns off and the energy stored in transformer T2 causes current to flow through winding L6, turning transistor Q2 on. When transistor Q2 is on, the current flowing through winding L8 causes current to flow through winding L6, which provides continued base drive current for transistor Q2. In addition, a winding L4 magnetically coupled to output transformer T1 is coupled to winding L6 via a diode network 13 and a resistor R1. Winding L4 provides additional base drive current for transistors Q1 and Q2. Other features of this prior art power supply are explained in greater detail in the above-referenced Boschert patent.
Although the inverter shown in the '445 patent provides a square wave output across load resistor RL, under certain circumstances, inverter 10 exhibits various undesirable phenomena. For example, at the end of a switching interval, after transistor Q1 turns off, the voltage at the collector of transistor Q1 increases and the voltage present at the collector of transistor Q2 approaches ground. Because of the inductive nature of transformer T1, the voltage at the collector of transistor Q2 continues to fall, even below ground, until the current flowing into the base of transistor Q2 equals the magnetizing current drawn into transformer T1. When this condition exists, current flow across the base-collector junction of transistor Q2 causes the potential present at the base of transistor Q2 to drop as well. As the voltage at the base of transistor Q2 drops, because of the magnetic coupling between windings L6 and L5, the voltage at the base of transistor Q1 starts to rise. If it rises sufficiently for transistor Q1 to turn on, then the undesirable result of having transistor Q1 turn on with a large voltage across its collector and emitter will occur, potentially causing damage to transistor Q1. This condition may occur transiently or in a "ringing" fashion. In addition, this phenomenon occurs when transistor Q2 turns off and the voltage present at the collector of transistor Q1 falls below ground.
In addition, the frequency of inverter 10 depends in part on the time it takes for saturable core transformer T2 to go into saturation, which depends on the voltage appearing across windings L5 and L6. If the above-mentioned effect causes the voltage across windings L5 and L6 to be lower than normal during the beginning of each half-cycle, then the saturation time for transformr T2 will increase, causing the frequency of oscillation of inverter 10 to fall.